The present invention relates to a flash vaporizer for delivering vaporized reactants to an oxidation/flame hydrolysis glass soot deposition system, and particularly to improved delivery and flash vaporization chamber means for vaporizing TiCl.sub.4 at a controlled rate as a thin film.
In order to enhance the fatigue resistance or other mechanical properties of an optical fiber, or to effect a change in the index of refraction of a vapor deposited soot preform for the optical fiber, the chemical composition of the vapors which are reacted to form the deposited soot may be varied. In the soot deposition process, the vapor mixture is oxidized/hydrolyzed at a burner to form a glass soot which is subsequently fused to form a high quality glass. Typically, SiCl.sub.4 is the primary vapor constituent. One or more additional vapors can be supplied to the oxidation/flame hydrolysis burner, the one or more vapors comprising chemical precursors of dopants whose presence affects the properties of the glass being formed.
In order to form a soot preform having generally consistent properties, and to assure an even distribution of the glass forming soot, it is necessary to supply the burner with a substantially constant flow of vaporized source material entrained in a carrier gas, such as O.sub.2. Accordingly, systems have been devised for controlling the carrier gas flow and the rate at which source material is vaporized and entrained into the carrier gas.
The reactant flow is typically measured in the vapor state. Alternative systems have been disclosed which meter the reactants in their liquid state, and thereafter vaporize or nebulize the reactants prior to their introduction into an oxidation/flame hydrolysis burner.
U.S. Pat. No. 4,173,305, issued on Nov. 6, 1979 and U.S. Pat. No. 4,230,744 issued on Oct. 28, 1980, disclose a system for precisely controlling liquid source materials and delivering the liquid to a mixer and nebulizer followed by delivery to an oxidation/flame hydrolysis burner. Each source material is maintained in liquified form within a reservoir and the liquid is transferred by means of an individually controlled metering pump to a mixing stage and a nebulizing stage. Oxygen is delivered to the nebulizer through a mass flow controller to be intermixed with the liquid reactants during the nebulizing stage. The nebulized vapors are delivered to an oxidation/flame hydrolysis burner. An O.sub.2 carrier gas is also introduced into the burner prior to being conveyed to the discharge means.
U.S Pat. No. 4,314,837, issued on Feb. 9, 1982 to M. G. Blankenship, discloses a method for delivering vapor source materials to an oxidation/flame hydrolysis burner. The Blankenship system comprises first and second enclosed reservoirs, each containing a liquid reactant which is a precursor of a dopant to be included in the soot preform. Each reservoir comprises heating means to heat the liquid contained therein to a temperature sufficient to maintain a predetermined minimum vapor pressure. Coupled to each reservoir is a mass flow controller for delivering vapors disposed within each reservoir at a controlled flow rate. After passage through the mass flow controller the vapors are combined with an O.sub.2 carrier gas prior to being conveyed to the burner means. A significant problem associated with the aforementioned system is the individual controlled metering means coupled to each reservoir. The mass flow controller is inoperable with liquids having a high boiling point.
U.S. Pat. No. 4,529,427, issued on July 16, 1985 to W. G. French, discloses a method for delivering vaporous reactants to a vapor deposition means, in which the reactants are vaporized in a flash evaporation chamber. The liquid reactants are supplied to the flash evaporation chamber by metering pumps. Oxygen is also supplied to the flash evaporation chamber and intermixed with the vaporized reactants prior to delivery to the vapor deposition means. Although the liquid reactants are delivered to the flash evaporating chamber in controlled amounts, the liquid is sprayed onto a heating surface whereby immediate vaporization occurs, creating nucleate or film boiling. Although this method does avoid the metering of vaporized gases, the hot spots created in the flash evaporation chamber and the introduction of a carrier gas induce pressure oscillations.
The system described in Blankenship, U.S. Pat. No. 4,314,837 is constrained by the limits in temperatures and flow rates at which it could operate. The other prior art methods have suffered from various disadvantages, the most limiting of which has been the presence of pressure oscillations, due to nucleate or film boiling and due to the introduction of the carrier gas into the flash vaporization chamber.
In a prior art system developed by the assignee of the applicants, liquid TiCl.sub.4 was vaporized in a flash vaporizer. A rod in cylinder configuration was utilized with a gap between the rod and cylinder of approximately 0.040 inch, and O.sub.2 was supplied to the flash vaporizer along with liquid TiCl.sub.4. The flash vaporizer was fed by a 1/4 inch delivery tube. The TiCl.sub.4 stream accumulated in the flash vaporizer, resulting in insufficient heat transfer. The gap width and liquid TiCl.sub.4 /O.sub.2 flow were such that an uniform thin film was not created, resulting in pressure oscillations. The temperature in this configuration was maintained at about 220.degree. C. to about 260.degree. C., which is well above the boiling point of TiCl.sub.4, 136.degree. C., resulting in nucleate boiling of TiCl.sub.4. Unacceptable pressure oscillations occurred due the introduction of O.sub.2 into the flash vaporizer and nucleate boiling of the liquid TiCl.sub.4.
To overcome these disadvantages and others, in the present invention a first liquid reactant is delivered to the flash vaporization chamber to form a thin film, vaporized in the flash vaporizer, and mixed with oxygen after vaporization. Thereafter additional vaporized reactants are mixed with the vaporized first liquid prior to delivery to an oxidation/flame hydrolysis burner.
It is therefore an object of the present invention to control the temperature within the flash vaporization chamber to prevent nucleate boiling of the liquid which leads to pressure oscillations in the vapor flow.
Another object of the present invention is to provide an improved system for delivering reactants at high flow rates to an oxidation/flame hydrolysis burner for glass soot deposition.
Another object is to provide a liquid only flash vaporization chamber in which the flow of unvaporized liquid is undisturbed by vapor exiting the flash vaporization chamber.
Another object is to provide a method to reduce dopant concentration variations resulting from the use of pressurized gas to move liquid TiCl.sub.4 through the pressure detection means to the flash vaporization chamber.